1. Field of the Invention
The present invention relates to a load sensor for sensing a load, applied to a loading point of the pressure-receiving portion, by the deflection of a strain-generating portion which is provided with a strain-sensing element or in which the strain-sensing element is formed by printing.
2. Description of the Related Art
FIG. 17 is a plan view illustrating a prior-art load sensor of this type. FIG. 18 is a cross-sectional view taken along line 18xe2x80x9418 of FIG. 17. Referring to these figures, a base body of the load sensor is generally designated by reference numeral 1. The base body 1 is formed of a sheet of metal, and comprises a mount portion 2 on the side of a securing end and a pair of strain generating portions 4 that extend from the mount portion 2 along the rims on both sides of an opening 3 adjacent thereto. The base body 1 further comprises a link portion 5 for linking the free ends of the strain generating portions 4 together and a pressure-receiving portion 6 that projects in the shape of the tongue from the link portion 5 into the opening 3.
The base body 1 has a flat print surface on which an insulative-coated layer and a trace pattern (not shown in any figures) are formed by printing. Each of the strain generating portions 4 is reduced in thickness at a portion 4a near the mount portion 2 and at a portion 4b near the link portion 5. On the insulative-coated layer overlying the thin portions 4a, 4b, strain-sensing elements 7, 8 of a thick-film resistive material are formed by printing, respectively. Incidentally, the strain-sensing elements 7, 8, arranged at four points in total, are interconnected with the aforementioned trace pattern to constitute a Wheatstone bridge circuit. There are provided a pair of screw holes 2a bored in the mount portion 2, in each of which a bolt 9 is inserted. The head of the bolt 9 exerts a pressure on the circumferential portion of the screw hole 2a, thereby securing the mount portion 2 to an external support member 10. There is provided a loading point 6a on the tip portion of the tongue-shaped pressure-receiving portion 6. External application of a load to the loading point 6a will produce bending moments, opposite in direction to each other, on the thin portions 4a, 4b of each of the strain generating portions 4. This causes each of the strain generating portions 4 to be deformed slightly in the shape of a letter xe2x80x9cSxe2x80x9d. In other words, the application of a load to the loading point 6a from above will cause the thin portion 4a, the closer of the thin portions 4a, 4b in each of the strain generating portions 4 to the mount portion 2, to be deformed in an upward convex shape. In contrast, the thin portion 4b, the closer to the link portion 5, will be deformed in a downward convex shape. Accordingly, the strain-sensing element 7 on the portion 4a senses a tensile stress, whereas the strain-sensing element 8 on the portion 4b senses a compressive stress. The level of the strain, which is produced in the strain generating portions 4 and sensed with the strain-sensing elements 7, 8, makes it possible to determine the load acted upon the loading point 6a. 
Incidentally, also known is a load sensor, comprising a cantilever-shaped plate member with a strain-sensing element, in which a load is applied to the tip portion of the member. This load sensor is not capable of sensing stress at a plurality of points where bending moments opposite in direction to each other are produced, being therefore subject to a problem of providing an insufficient accuracy in comparison with the aforementioned load sensor.
The prior-art load sensor shown in FIG. 17 is subject to variations in performance since no consideration is given to the effects exerted by the fastening force applied to secure the mount portion 2. That is, the base body 1 is attached to the support member 10 using securing screw means such as the bolt 9, thereby causing the circumferential portion of the screw holes 2a of the mount portion 2 or the securing end portion of the base body 1, to be strongly pressurized onto the support member 10. The strong pressurization of the base body 1 formed of a sheet of metal will cause a radial stress to act also upon and thereby produce a deformation in a portion outside the directly pressurized portion. This deformation is apt to have an adverse effect on the characteristic of the neighboring strain-sensing element 7. In addition, since the deformation caused by the fastening force is slightly different from product to product, the strain-sensing element 7 delivers different outputs, varying from product to product, even under the same load applied to the loading point 6a. This makes it difficult to provide the desired reliability.
The prior-art load sensor shown in FIGS. 17 and 18 is fabricated through the process in which a trace pattern, the strain-sensing elements 7, 8 or the like are formed by printing on a flat print surface of the base body 1. Therefore, this makes the load sensor suitable for mass production at low costs. However, the print surface of the base body 1 is designed to accommodate all the components in the same plane including the pressure-receiving portion 6. This would cause the tongue-shaped pressure-receiving portion 6 to be subjected to deflections in the heating process for forming the trace pattern or the like by printing, thereby causing the tip portion of the pressure-receiving portion 6 having the loading point 6a to sit above the print surface in some cases. In those cases, being interfered with the tip portion of the pressure-receiving portion 6, the strain-sensing elements 7, 8 would not be printed smoothly, thereby raising the possibilities of impairing the sensing accuracy and reducing fabrication yields.
The present invention has been developed in view of the aforementioned circumstances of the prior art. It is therefore an object of the present invention to provide a highly reliable load sensor which does not allow the fastening force for attaching the securing end portion to have an adverse effect on the characteristic of the strain-sensing element and provides reduced variations in performance from product to product. It is another object of the present invention to provide a load sensor in which defective printing of strain-sensing elements caused by deflection of the pressure-receiving portion can be avoided and which is suitable for providing stabilized quality and improved fabrication yields.
To achieve the aforementioned objects, the present invention provides a load sensor comprising: a mount portion on one end of the load sensor, the mount portion having a screw hole for allowing securing screw means to be inserted therein, the securing screw means applying a pressure to a circumferential portion of the screw hole to thereby secure the mount portion to the securing screw means; strain-generating portions extending from the mount portion to the other end of the load sensor along side rims of an opening; a pressure-receiving portion, linked to the strain-generating portions at the other end, projecting in the shape of the tongue into the opening, having a loading point at an end point; strain-sensing elements, formed in the strain-generating portions, for sensing strain of the strain generating portions, the strain caused by a load applied to the loading point. The load sensor is adapted that at least part of the opening is interposed between the center of the screw hole and the strain-sensing elements.
This allows the deformation caused by a strong fastening force for pressurizing the circumferential portion of the screw hole to be blocked by the opening and thereby not to have a direct effect on the strain-sensing elements. The characteristic of the strain-sensing elements is therefore prevented from varying from product to product. The load sensor may comprises, as the strain-sensing elements, a first strain-sensing element formed at an area of the strain-generating portions near the mount portion and a second strain-sensing element formed at an area of the strain-generating portions near a link portion with the pressure-receiving portion. In this configuration, the load sensor may be adapted to sense the tensile stress and the compressive stress of the strain-generating portion which is deflected slightly in the shape of a letter xe2x80x9csxe2x80x9d when a load is acted upon the loading point of the pressure-receiving portion. This makes it possible to provide a highly reliable load sensor which has a good sensing accuracy and reduced variations in performance from product to product. The load sensor may be adapted that the distance between the loading point and the first strain-sensing element is generally equal to the distance between the loading point and the second strain-sensing element. This allows the tensile stress sensed by the first strain-sensing element to be generally equal in magnitude to the compressive stress sensed by the second strain-sensing element. This preferably simplifies the configuration of the bridge circuit containing the strain-sensing elements.
In the aforementioned configuration, the mount portion, the strain-generating portion, and the pressure-receiving portion may be provided on one piece of plate-shaped member. This allows the base body of the load sensor to be easily machined and a bridge circuit containing the strain-sensing elements to be easily formed on the base body by a method such as printing, thereby making it possible to fabricate the load sensor at low costs. The strain-generating portion may be made thinner than the mount portion and the pressure-receiving portion in thickness, thereby allowing the strain-generating portion to deflect easily and thus provide improved accuracy readily.
In the aforementioned configuration, the load sensor may be provided with a projection, projecting in the direction of thickness of the pressure-receiving portion, on the tip portion of the pressure-receiving portion, and the top portion of the projection acting as the loading point. This ensures that the load of a measurement object can be applied to the loading point even when the measurement object is more or less misaligned with the load sensor in the direction of the surface of the load sensor. Therefore, this makes it possible for the load sensor to avoid malfunctioning even with a slight error in the position of the attachment of the load sensor. The load sensor may be adapted that the projection is provided on both the front and reverse surfaces of the tip portion of the pressure-receiving portion, thereby allowing the load sensor to measure the load of a measurement object present on either the front or reverse side of the pressure-receiving portion.
The present invention also provides a load sensor comprising: a mount portion on one end of the load sensor, the mount portion being secured to an external device; an opening adjacent to the mount portion; strain-generating portions extending from the mount portion to the other end of the load sensor along the side rims of the opening, the strain-generating portions having at least one flat print surface; a pressure-receiving portion, linked to the strain-generating portions at the other end, projecting in the shape of the tongue into the opening; and strain-sensing elements, formed by printing on the print surface of the strain-generating portions, for sensing strain of the strain generating portions, the strain caused by a load applied to the pressure-receiving portion. The load sensor is adapted that under no load applied to the pressure-receiving portion, the tip portion of the pressure-receiving portion stays below a plane containing the print surface in the direction of thickness of the pressure-receiving portion.
As described above, the tip portion of the pressure-receiving portion may be adapted to stay in advance below a plane containing the print surface in the direction of thickness of the pressure-receiving portion. Accordingly, even when the pressure-receiving portion is deflected in the heating process during fabrication, there is no worry that the tip portion of the pressure-receiving portion stays above the print surface of the strain generating portions. It is thereby possible to avoid defective printing of the strain-sensing elements caused by the deflection of the pressure-receiving portion.
Incidentally, the tip portion of the pressure-receiving portion may be adapted to stay in advance below the plane containing the print surface of the strain-generating portions. In this case, for example, the tip portion of the pressure-receiving portion may be made thinner than the proximal end thereof in thickness. Alternatively, a pressure-receiving portion having a uniform thickness may be curved such that the tip portion thereof is gradually separated from the plane containing the print surface.